1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to perpendicular magnetic recording (PMR) media.
2. Statement of the Problem
One type of recording media presently used in magnetic recording/reproducing apparatuses is longitudinal magnetic recording media. Longitudinal magnetic recording media includes a magnetic recording layer having an easy axis of magnetization parallel (with a random way in two-dimension) to the substrate. The easy axis of magnetization is the crystalline axis that is aligned along the lowest energy direction for the magnetic moment. Another type of recording medium is perpendicular magnetic recording (PMR) media. PMR media includes a magnetic recording layer having an easy axis of magnetization oriented substantially perpendicular to the substrate. Hexagonal Close Packed (HCP) Co-alloys are typically used as the magnetic recording layer for both longitudinal and perpendicular recording. The easy axis of magnetization for these materials lies along the c-axis.
PMR media is generally formed on a substrate with a soft magnetic underlayer (SUL), one or more underlayers, and a perpendicular magnetic recording layer. The soft magnetic underlayer (SUL) serves to concentrate a magnetic flux emitted from a main pole of a write head and to serve as a flux return path back to a return pole of the write head during recording on the magnetic recording layer. The underlayers serve to control the size of magnetic crystal grains and the orientation of the magnetic crystal grains in the magnetic recording layer. The underlayers also serve to magnetically de-couple the SUL and the magnetic recording layer. The magnetic recording layer is the layer in which bits are stored based on the orientation of the magnetization of individual magnetic grains.
Coercivity and signal-to-noise ratio (SNR) of PMR media are related to the magnetic grain separation (as well as magnetic grain size) in the magnetic recording layer. The initial growth of the magnetic recording layer contributes to the degree of isolation between the magnetic grains and to the size of magnetic grains. Although an increase in the isolation between the magnetic grains and a decrease in size of magnetic grain lead to a higher SNR, over-isolation and too-small grain can result in thermal instability of the magnetic recording layer. If the thermal instability is too high, there may be enough thermal energy available during operation to reverse the magnetization within a region of the magnetic recording layer, destroying the data stored within the region. Thus, it would be desirable to increase the isolation between the magnetic grains and reduce the grain size in the magnetic recording layer while controlling the grain isolation and size to maintain thermal stability.